This invention relates to the manufacture of moulded polyurethane (or moulded polyurethane containing products), especially thermoplastic products such as elastomers, flexible foam, and rigid foam using gas assisted injection moulding (herein after referred to as xe2x80x9cGAIMxe2x80x9d).
Thermoplastic polyurethanes (herein after referred to as xe2x80x9cTPUsxe2x80x9d or xe2x80x9cTPUxe2x80x9d) are well-known thermoplastic elastomers. TPUs exhibit very high tensile and tear strength, high flexibility at low temperatures, and extremely good abrasion and scratch resistance. TPUs also have a high stability against oil, fats and many solvents, as well as stability against UV radiation. Because of these desirable features, TPUs are employed in a number of end use applications, such as in the automotive and the footwear industries.
Because many of the end use applications require that the TPU be light (i.e. have a low density), there is always a demand for lighter TPUs. Developing such low density TPUs presents many technical challenges because the desirable physical properties of TPUs are often sacrificed in order to reach this lower density.
It is already known to produce soles and other polyurethane parts by a poly-addition reaction of liquid reactants resulting in an elastic solid moulded body. The reactants used are polyisocyanates and polyesters or polyethers containing OH-groups. Foaming is affected by adding a liquid of low boiling point or by means of CO2, thereby obtaining foam with at least partially open cells. Reducing the weight of the materials by foaming the TPU has not given satisfactory results.
Attempts to foam TPU using well-known blowing agents, such as azodicarbonamides (exothermic) or sodiumhydrocarbonate (endothermic) based products were not successful for mouldings with reduced densities below 800 kg/m3. With endothermic blowing agents, a good surface finish can be obtained but the lowest density achievable is about 800 kg/m3. Also, the processing is not very consistent and results in long demoulding times. Very little or no foaming is obtained at the mould surface due to a relatively low mould temperature, resulting in a compact, rather thick skin and a coarse cell core. By using exothermic blowing agents, a lower density foam (down to 750 kg/m3) with very fine cell structure can be achieved, but the surface finish is not acceptable for most applications and demould time is even longer.
Therefore, it is clear that there is a continuous demand for low density TPUs that have improved skin quality and can be produced with reduced demould times.
Therefore, it is an object of the present invention to provide low density TPUs that have improved skin quality and can be produced with reduced demould times.
It has been surprisingly found that moulded polyurethane containing products that are manufactured using gas assisted injection moulding meet the above objectives. Demould times are significantly reduced and the process can be carried out at lower temperatures, resulting in a better barrel stability. In addition, further density reduction is obtained while maintaining or improving the skin quality and demould time. The low density thermoplastic polyurethanes thus obtained (density not more than 800 kg/m3) have a fine cell structure, a uniform density profile, very good surface appearance, a skin having uniform thickness and show comparable physical properties to conventional PU which renders them suitable for a wide variety of applications.
Particularly, the invention provides TPU products having outstanding low temperature dynamic flex properties and green strength at the time of demould, at density 800 kg/m3 and below. The term xe2x80x9cgreen strengthxe2x80x9d, as is known in the art, denotes the basic integrity and strength of the TPU at demould. The polymer skin of a moulded item, for example, a shoe sole and other moulded articles, should possess sufficient tensile strength and elongation and tear strength to survive a 90 to 180 degree bend without exhibiting surface cracks. The prior art processes often require 5 minutes minimum demould time to attain this characteristic. The present invention provides a significant improvement in minimum demould time, as a demould time of 2 to 3 minutes is achievable.
There is different equipment capable of pressurising a gas such that it may be injected into a moulding process. Examples are cited in EP 467 565 and EP 648 157. However, none of these have been successfully implemented for the manufacturing of moulded polyurethane, especially thermoplastic products.
In one embodiment, the present invention relates to a process for making moulded thermoplastic polyurethane products using gas assisted injection moulding. According to another embodiment, a process for making moulded polyurethane products is provided by using a gas counter pressure together with the gas injection moulding. According to yet another embodiment, a process for making moulded polyurethane products is provided by using gas assisted injection moulding in the presence of expandable microspheres.